Control of energy storage to reduce electric power system off-nominal frequency deviations

ABSTRACT

A hybrid power generation system is formed by the combination of an energy storage system (ESS) and a rotating synchronous power generator (SPG). Energy is stored in or released from the ESS in response to measurements of the at least one angle parameter, selected from rotor, torque, or power angle of the SPG, to provide active frequency damping of electrical power output. The control of ESS energy exchange increases the stabilizing impact of the SPG inertia on the frequency of electricity in an electrical network or power grid. The hybrid power generation system can have an effective equal area criterion for stability limit that is greater than that of the SPG operating without the ESS. The hybrid power generation system can enable the electrical network to have a greater proportion of variable or distributed energy resource (DER) power generation systems without otherwise exceeding stability limits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/857,663, entitled “CONTROL OF ENERGY STORAGE TO REDUCE ELECTRIC POWERSYSTEM OFF-NOMINAL FREQUENCY DEVIATIONS,” filed Jun. 5, 2019, which isincorporated by reference herein in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under ContractDE-AC05-76RL01830 awarded by the U.S. Department of Energy (DOE). TheGovernment has certain rights in the invention.

FIELD

The present disclosure relates generally to electric power generationsystems, and more particularly, to control of energy storage andtransmission to control operational deviations of electric powergeneration systems.

BACKGROUND

Alternating current (AC) electric power generated by power generationsystems can be transmitted to end users (e.g., power consumers orcustomers) via a power grid or electrical network, which includestransmission lines, substations, distribution lines, etc. The electricalnetwork is designed to supply electricity at a relatively constantnominal frequency (e.g., 60 Hz in the North America). Temporalvariations or disturbances within the network (e.g., failure oftransmission lines or substations) or at input/output of the network(e.g., failure of a power generation system, or changes in user demand)can cause fluctuations in the nominal frequency of the suppliedelectricity. Historically, the majority of power generation systemscoupled to the electrical network have been rotating synchronousgenerators, which provide physical inertia that helps to stabilize thefrequency of electricity within the electrical network despite suchdisturbances.

Increasingly, electrical networks include renewable power generationsystems (e.g., wind and photovoltaic) that produce power output ofvarying magnitude and/or timing. Modern electrical networks may alsoinclude distributed energy resources (DER), such as photovoltaicinstallations at a particular building or site. However, such variableand DER power generation systems may lack grid-stabilizing physicalinertia or provide physical inertia substantially less than that ofconventional rotating synchronous generators. Thus, as the percentage ofelectricity that is produced by rotating synchronous generatorsdecreases, the electrical network may be more susceptible frequencydestabilization after a disturbance.

Energy storage systems (ESSs) have been used to improve frequencystability by storing energy from or releasing energy to the electricalnetwork. However, control of energy exchange between the ESSs and theelectrical grid has employed droop-control schemes, where the exchangeof energy is proportional to power system frequency. Yet thestabilization of network frequency using energy storage has not beenwidely implemented in industry, nor has there been a consensus onappropriate technical designs to achieve frequency stabilization usingenergy storage. Moreover, since conventional control of the ESS is basedon frequency, the electrical network must deviate from nominal operatingfrequency before any energy storage or release is initiated, which maylead to delays in addressing system disturbances and/or undesirableoscillations around nominal frequency.

Embodiments of the disclosed subject matter may address one or more ofthe above-noted problems and disadvantages, among other things.

SUMMARY

Certain embodiments of the disclosed subject matter are directed to ahybrid power generation system formed by the combination of an energystorage system (ESS) and a rotating synchronous power generator (SPG).The hybrid power generation system can store energy in or release energyfrom the ESS based on operation of the rotating SPG in order to increasethe stabilizing impact of SPG inertia on the frequency of electricity inan electrical network coupled to the hybrid power generation system. Forexample, energy exchange with the ESS can be controlled in response tomeasurements of at least one angle parameter (e.g., measurements ofrotor angle, torque angle, and/or power angle) of the SPG to provideactive frequency damping of the electrical power output of the SPG. Insome embodiments, the control based on SPG angle parameter measurementsmay compensate for disturbances before frequency fluctuations arise orare detected in the electrical network. In some embodiments, the energyexchange with the ESS can be controlled in response to measurements ofonly one of the rotor angle, torque angle, or power angle. In someembodiments, the energy exchange with the ESS can be controlled inresponse to measurements of any two, or all, of the rotor angle, torqueangle, or power angle.

While the ESS may be a static device with no rotation of its own, theESS can be considered to add “synthetic inertia” to the SPG, therebyamplifying its stabilizing capacity and/or frequency damping ability.The combination of the ESS with the SPG can thus extend (e.g., increase)the effective equal area criterion for stability limit as compared tothe SPG operating alone. Since frequency stabilization is improved bythe hybrid power generation system, the electrical network may be ableaccommodate a greater percentage of variable (e.g., renewables) or DERpower generation systems without otherwise exceeding stability limits.

In one or more embodiments, an apparatus can comprise an ESS and acontrol system. The ESS can be coupled to an output of an electricalpower generator. The control system can have a signal output coupled tothe ESS that causes the ESS to store or release energy based on at leastone angle parameter, the angle parameter being selected from rotorangle, torque angle, or power angle of the electrical power generator.

In some embodiments, the control system can comprise one or moreprocessors, an input/output interface, and a computer-readable storage.The input/output interface can be situated to control the energy storagesystem. The computer-readable storage can store computer-executableinstructions that, when executed by the one or more processors, causethe one or more processors to collectively control the ESS to store orrelease energy based on the at least one angle parameter.

In some embodiments, the control system can be configured to controlenergy storage or release by the ESS to provide active frequency dampingof electrical power output.

In some embodiments, the control system can be configured to controlenergy storage or release by the ESS so as to stabilize a frequency ofelectrical power output at a nominal value or within a frequency rangearound the nominal value.

In some embodiments, the ESS can comprise an inverter or flexiblealternating current transmission system (FACTS) and at least one of abattery, a capacitor, a flywheel, a stationary power source (e.g., fuelcell), a pumped fluid storage, or a thermal energy storage.

In some embodiments, the electrical power generator can comprise atleast one of a steam generator, a combustion turbine generator, acombined cycle generator, a hydroelectric generator, or a diesel ornatural gas engine generator.

In some embodiments, the control system can have a signal input coupledto the electrical power generator to receive sensor signals indicativeof the at least one angle parameter of the electrical power generator.

In some embodiments, the control system can be configured to control theESS to store energy in response to a positive change of the at least oneangle parameter over time, and to control the ESS to release storedenergy in response to a negative change of the at least one angleparameter over time.

In some embodiments, the control system can be configured to control theESS such that an amount of energy stored or release is a function of atleast one of a magnitude of the at least one angle parameter, adifference between the magnitude of the at least one angle parameter anda nominal value, and/or a rate of change of the at least one angleparameter.

In some embodiments where the energy exchange with the ESS is controlledin response to two or more measurements, the control system is furtherconfigured to control the ESS additionally in response to a positivechange, a negative change, a magnitude, or a difference in magnitude of:only one of the measurements, to two or more of the measurements, or allof the measurements.

In some embodiments, the ESS can be coupled to the output of theelectrical power generator via a local bus.

In one or more embodiments, a method, for operating an ESS coupled to anelectrical power generator, can comprise, in a first operation mode,controlling storage of energy in or release of stored energy from theESS based on a measurement of at least one angle parameter, the angleparameter being selected from rotor angle, torque angle, or power angleof the electrical power generator.

In some embodiments, the controlling can provide active frequencydamping of electrical power from the electrical power generator.

In some embodiments, the controlling can be such that a frequency ofelectrical power output from a combination of the ESS, and/or such thatthe electrical power generator is stabilized at a nominal value orwithin a frequency range around the nominal value.

In some embodiments, the ESS can comprise at least one of a battery, acapacitor, a flywheel, a stationary power source (e.g., fuel cell), apumped fluid storage, or a thermal energy storage.

In some embodiments, the method can further comprise receiving at leastone sensor signal from the electrical power generator or a powermanagement unit thereof. The at least one sensor signal can beindicative of the at least one angle parameter, or can be indicative ofchanges of the at least one angle parameter over time. The controllingcan be responsive to the at least one sensor signal.

In some embodiments, the controlling can comprise controlling the ESS tostore energy in response to a positive change of at least one angleparameter during a measurement time interval, and/or controlling the ESSto release stored energy in response to a negative change of the atleast one angle parameter during the measurement time interval.

In some embodiments, the controlling can be such that an amount ofenergy stored in or release from the ESS is a function of at least oneof a magnitude of the at least one angle parameter, a difference betweenthe magnitude of the at least one angle parameter and a nominal value,and/or a rate of change of the at least one angle parameter.

In some embodiments, the method can further comprise, in a secondoperation mode, controlling the storage of energy in or release ofstored energy from the ESS based on power frequency variations in anelectrical network coupled to the electrical power generator. The ESScan be operated in the second operation mode in response to an absenceof the measurement of at least one angle parameter for the electricalpower generator, or to the measurement of the at least one angleparameter being outside a predetermined range.

In some embodiments, in the second operation mode, the controlling canbe such that an amount of energy stored in or release from the ESS isproportional to a frequency of the power in the electrical network or adifference between the frequency of the power and a nominal value.

In some embodiments, the method can further comprise, prior toinitiation of the first operation mode, determining that a frequency ofpower in an electrical network coupled to the electrical power generatoris outside of predetermined range.

In some embodiments, the electrical power generator can be a synchronouspower generator comprising at least one of a simple cycle steamgenerator, a combustion turbine generator, a combined cycle generator, ahydroelectric generator, or a diesel or natural gas engine generator.

In one or more embodiments, a control system can comprise one or moreprocessors and computer-readable storage media. The computer-readablestorage media can store computer-instructions that, when executed by theone or more processors, cause the one or more processors to perform anyof the disclosed methods for operating an ESS coupled to an electricalpower generator.

In some embodiments, the instructions stored by the computer-readablestorage media comprise instructions that cause the one or moreprocessors to receive one or more signals indicative of the at least oneangle parameter, or of changes in the at least one angle parameter overa time period.

In some embodiments, the instructions stored by the computer-readablestorage media can further comprise instructions that cause the one ormore processors to generate one or more first control signals for theESS that cause the ESS to store energy in response to a positive changein the at least one angle parameter during the time period.

In some embodiments, the instructions stored by the computer-readablestorage media can further comprise instructions that cause the one ormore processors to generate one or more second control signals for theESS that cause the ESS to release stored energy in response to anegative change in the at least one angle parameter during the timeperiod.

In one or more embodiments, a hybrid power generation system cancomprise an electrical power generator and a frequency damping unit. Theelectrical power generator can be configured to produce alternatingcurrent (AC) electrical power for an electrical network. The frequencydamping unit can be coupled to the electrical power generator. Thefrequency damping unit can comprise an ESS and a controller. Thecontroller can be configured to control the ESS to modulate a combinedpower output of the electrical power generator and the ESS based on atleast one angle parameter, the angle parameter being selected from rotorangle, torque angle, or power angle of the electrical power generator.

In some embodiments, the electrical power generator can have a physicalinertia, and the frequency damping unit can be configured to addsynthetic inertia to the physical inertia of the electrical powergenerator.

In some embodiments, the synthetic inertia added by the frequencydamping unit can act to stabilize a frequency of the combined poweroutput at a nominal value of the electrical network or within afrequency range around the nominal value.

In some embodiments, an effective equal area criterion for stabilitylimit of the hybrid power generation system is greater than an effectiveequal area criterion for stability limit of the electrical powergenerator operating without the frequency damping unit.

In some embodiments, the hybrid power generation system can furthercomprise one or more sensors configured to measure the at least oneangle parameter of the electrical power generator and to generate one ormore sensor signals in response to the at least one measured angleparameter. The controller can be configured to control the ESS based onthe one or more sensor signals.

In some embodiments, the electrical power generator can be a synchronouspower generator comprising at least one of a simple cycle steamgenerator, a combustion turbine generator, a combined cycle generator, ahydroelectric generator, a diesel engine generator, or a natural gasengine generator.

In some embodiments, the ESS can comprise at least one of a battery, acapacitor, a flywheel, a stationary power source (e.g., fuel cell), apumped fluid storage, or a thermal energy storage.

In some embodiments, the controller can be configured to control the ESSto store energy in response to a positive change of the at least oneangle parameter during a time period, and/or control the ESS to releasestored energy in response to a negative change of the at least one angleparameter during the time period.

In some embodiments, the controller can be configured to control the ESSsuch that an amount of energy stored in or released by the ESS is afunction of at least one of a magnitude of the at least one angleparameter of the electrical power generator, a difference between themagnitude of the at least one angle parameter and a predetermined value,and/or a rate of change of the at least one angle parameter.

In some embodiments, the frequency damping unit can be coupled to theelectrical power generator by one or more local buses, and theelectrical power generator can be coupled to the electrical network byone or more power transmission lines.

In one or more embodiments, an electrical power system can comprise oneor more hybrid power generation systems, one or more variable orasynchronous (V/A) power generation systems, and an electrical network.The one or more hybrid power generation systems can be any of thedisclosed hybrid power generation systems. The electrical network can becoupled to the hybrid and V/A power generation systems. The electricalnetwork can be configured to transmit power from the power generationsystems to one or more end users.

In some embodiments, each frequency damping unit can be coupled to thecorresponding electrical power generator by one or more local buses,and/or each of the hybrid and V/A power generation systems can becoupled to the electrical network by one or more respective powertransmission lines.

In some embodiments, the one or more V/A power generation systems cancomprise at least one of a wind turbine or a photovoltaic device.

This summary is not intended to identify key features or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter. Objects and advantages ofembodiments of the disclosed subject matter will become apparent fromthe following description when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating aspects of anexemplary hybrid generation system, according to one or more embodimentsof the disclosed subject matter.

FIG. 2A is a graph that shows time variation of rotor angle of a powergeneration system and a generalized control scheme for an energy storagesystem, according to one or more embodiments of the disclosed subjectmatter.

FIG. 2B illustrates the concept of extension of a power generationsystem's equal-area stability limits by operation of an energy storagesystem, according to one or more embodiments of the disclosed subjectmatter.

FIG. 3 is a process flow diagram of an exemplary method involving ahybrid generation system, according to a first embodiment of thedisclosed subject matter.

FIG. 4 is a process flow diagram of an exemplary method involving ahybrid generation system, according to a second embodiment of thedisclosed subject matter.

FIG. 5 is a process flow diagram of an exemplary control method for anenergy storage system, according to one or more embodiments of thedisclosed subject matter.

FIG. 6 is a process flow diagram of another exemplary control method foran energy storage system, according to one or more embodiments of thedisclosed subject matter.

FIG. 7A is a graph illustrating measured rotor angle over time for asynchronous power generator of a hybrid generation system following apower system disturbance.

FIG. 7B is a graph illustrating output over time of an electricalstorage system of the hybrid generation system following the powersystem disturbance.

FIG. 8 depicts a generalized example of a suitable computing environmentin which the described innovations may be implemented.

DETAILED DESCRIPTION

As renewable resources displace traditional synchronous generation,there may be times when there is insufficient system inertia tostabilize system frequency in the electrical network following a powersystem disturbance. Accordingly, embodiments of the disclosed subjectmatter provide a hybrid generation system that increases the inertialimpact of a rotating synchronous power generator (SPG) and therebyincreases the overall power system frequency stability. The hybridgeneration system can include the SPG and an energy storage system (ESS)coupled thereto. The ESS can be controlled to store energy or releasestored energy in response to operation of the SPG, for example, based onat least one measured angle parameter of the rotating SPG. As usedherein, angle parameter refers to a parameter selected from rotor angle,torque angle, or power angle of the rotating SPG. Such control canimprove the frequency stability of the hybrid generation system and/orthe electrical network coupled thereto and/or reduce a time for thesystem frequency to recover from a disturbance. The combination of theESS with the SPG can thus extend the effective equal area criterion forstability limit as compared to the SPG alone. Moreover, since frequencystabilization is improved by the hybrid power generation system, theelectrical network may be able accommodate a greater percentage ofvariable (e.g., renewable resources) or distributed energy resource(DER) power generation systems without otherwise exceeding stabilityregulations.

FIG. 1 illustrates an exemplary electric power system 100 that includesa hybrid generation system 102 according to one or more embodiments. Theelectric power system 100 includes an electrical network 140 (e.g.,power grid) that conveys generated electricity to one or more powerconsumers or customers 150, for example, via a distribution line 148. Inthe illustrated example, the electrical network 140 receives electricpower generated by rotating SPGs 142 a, 142 b via respectivetransmission lines 146 and electric power generated by variable orasynchronous (V/A) power generators 144 a, 144 b via respectivetransmission lines 146. For example, the V/A power generators cangenerate electric power using a renewable resource (e.g., wind turbinesin 144 a and solar photovoltaic in 144 b) and/or be a DER system. TheSPGs 142 a, 142 b can be conventional power generation systems, such assteam generators (e.g., fossil fuel, solar thermal, or geothermal),combustion turbine generators, combined cycle generators, hydroelectricgenerators, and diesel or natural gas engine generators. The electricalnetwork can also receive electric power from the hybrid generationsystem 102 via transmission line 138. The hybrid generation system 102can include a rotating SPG 106 and a frequency damping unit 104electrically coupled thereto. The SPG 106 and/or SPGs 142 a, 142 b canbe any type of conventional rotating power generation system, such as,but not limited to, simple cycle steam generators (e.g., fossil fuel,solar thermal, or geothermal), combustion turbine generators, combinedcycle generators, hydroelectric generators, and diesel or natural gasengine generators.

In some embodiments, the SPG 106 and the frequency damping unit 104 ofthe hybrid generation system are in close proximity to each other so asto be co-located, e.g., provided at a same site or installation. Thus,electric power from both the SPG 106 and the frequency damping unit 104can be provided to the electrical network 140 via the same transmissionline 138. For example, SPG 106 can transmit generated electric power viaan output power line 124 connected to local bus 126. Frequency dampingunit 104 can also be connected to local bus 126 via input/output powerline 116. The local bus 126 can be connected to transmission line 138and thereby to the electrical network 140. Via power line 116, local bus126, and transmission line 138, the frequency damping unit 104 can thusreceive energy from the SPG 106 and/or the electrical network 140 forstorage and release stored energy to the electrical network 140.

The frequency damping unit 104 can include an ESS 108 and a controller114 operatively coupled thereto by signal line 120. The ESS 108 caninclude at least one energy storage 112 and at least one powerelectronics 110 connected to the energy storage 112 via power line 118.Each energy storage 112 can be a device capable of directly orindirectly storing electrical energy and of subsequently releasingstored energy as electricity. Each power electronics 110 can be a devicecapable of converting electrical power at system frequency (e.g., 60 Hz)for storage by energy storage 112 (e.g., DC voltage) and of convertingstored energy from the energy storage 112 to electrical power at systemfrequency. The energy storage 112 and/or the power electronics can alsohave a relatively fast response time (e.g., less than is response time).For example, each energy storage 112 and power electronics 110 can havea response time on the order of 50 ms. In some embodiments, the energystorage 112 can include one or more of a battery (e.g., lithiumbattery), capacitor, flywheel, stationary power source (e.g., fuelcell), pumped fluid storage (e.g., pumped hydro storage or compressedair energy storage), or thermal energy storage. In some embodiments, thepower electronics 110 can include one or more of an inverter (e.g.,UL-1741-SA compliant) and a flexible AC transmission system (FACTS)device. Other types of fast-responding storage devices andgrid-connecting power electronics are also possible according to one ormore contemplated embodiments.

The controller 114 can be configured to dynamically controlbi-directional energy exchange of the ESS 108 to provide frequencystabilization of the combined electric power output from hybridgeneration system 102 to electrical network 140 and/or of the electricpower within electrical network 140. For example, the controller 114 cancontrol the ESS 108 to store energy in energy storage 112 or to releasestored energy from energy storage 112 based on operation of SPG 106. Insome embodiments, the controller 114 can receive one or more signalsfrom a control or monitoring unit of the SPG 106, which signals providean indication of operation of the SPG 106. For example, the controller114 can receive signals from SPG 106 via signal line 122. In someembodiments, the controller 114 can directly monitor operation of theSPG 106, for example, where the controller 114 and the control unit ofthe SPG 106 are integrated together.

The SPG 106 may have one or more sensors that monitor operation thereofand generate one or more signals for use by controller 114 incontrolling ESS 108. For example, the sensor(s) can monitor at least oneangle parameter, such as rotor angle 136 of the SPG 106. Rotation ofrotor 128 within stator 130 is used to generate the AC electric poweroutput to power line 124 by SPG 106. The direction 132 of the rotatingelectric field of the rotor 128 is displaced from the direction 134 ofthe rotating electric field of the stator 130. The angle between therotor electric field direction 132 and the stator electric fielddirection 130 defines rotor angle 136. In other embodiments, the SPG 106may sense a surrogate variable that can be used to determine or estimaterotor angle 136. Alternatively or additionally, the sensor(s) canmonitor torque angle (i.e., the angle between rotor flux and statorflux) or power angle (i.e., the angle between the center line of theexcitation filed of the SPG and the center line of the reaction field ofthe stator). For a single rotating generator, however, the rotor,torque, and power angles are generally the same, and thus any or all ofthese angles can be used as the angle parameter(s) in controlling theenergy exchange of ESS 108 by controller 114.

When the electrical network 140 is subjected to a system disturbance(e.g., failure of a power generator, change in consumer demand, failureof parts of the electrical network, etc.) that causes a deviation of thepower system frequency from its nominal value, the deviation isreflected in a change of the angle parameter (e.g., rotor angle) of therotating SPG. In some examples, the rotor angle of the rotating SPG maybegin to change before power system frequency deviates from its nominalvalue. Using the angle parameter measurements from the SPG 106, thecontroller 114 can control ESS 108 to preemptively address the frequencydeviations (e.g., eliminate frequency fluctuations or at least maintainfluctuations to within an acceptable range) and/or to quickly return thesystem to nominal frequency (e.g., minimize, or at least reduce, a timeto return to nominal and/or a number of frequency oscillations aboutnominal).

Measurements for control by controller 114 can be time-synchronized andmay have relatively high resolution (e.g., 30-40 measurements persecond) to allow the hybrid generation system 102 to respond todisturbances in real-time or near real-time (e.g., less than is delay,and preferably less than 100 ms). For example, the controller 114 mayalso use synchrophasor technology, employing phasor measurement unit(PMU) data format, and can receive a time synchronization signal from aglobal positioning system (GPS) clock. Measurements of the angleparameter can be transmitted to the controller 114 in the PMU dataformat. In some embodiments, the controller 114 may also receive asignal indicative of system frequency, for example, when the ESS 108 iscontrolled to provide droop control. In such embodiments, the systemfrequency signal can also be in PMU data format.

The controller 114 can control ESS 108 to store energy in energy storage112 or to release energy from energy storage 112 as a function of theangle parameter (e.g., rotor angle), a change in the angle parameterwith respect to a nominal value or a deadband surrounding the nominalvalue, changes in the angle parameter over time, and/or a rate of changein the angle parameter over time. In some embodiments, the controller114 controls energy exchange of ESS 108 proportional to a magnitude ofangle parameter measurements or changes therein.

Alternatively or additionally, the controller 114 can be configured toemploy proportional-integral (PI) or proportional-integral-derivative(PID) control based on the angle parameter(s) of the SPG 106. Othercontrol schemes based on the angle parameter(s) of the SPG 106 are alsopossible according to one or more contemplated embodiments.

A system disturbance may cause oscillations of the rotor angle 136 aboutits nominal value and/or of the power system frequency about its nominalvalue. Control of the energy exchange of the ESS 108 by controller 114can act to eliminate these oscillations, or at least reduce a numberand/or magnitude of the oscillations. For example, the control of theenergy exchange of the ESS 108 can be effective to increase a systemfrequency damping (e.g., as measured by increased effective inertiaconstant (H) or inertia (MW-s)) and/or to actively dampen oscillationsso as to reduce undesirable oscillations about nominal frequency.

For example, FIG. 2A is a graph 200 illustrating an example of rotorangle variations in response to a system disturbance and correspondingenergy exchange of ESS 108. Before the disturbance, the rotor angleremains at a nominal value 202 a or in a control deadband 204surrounding nominal, and the ESS 108 can remain idle without any energyexchange. As SPG 106 initially attempts to compensate for thedisturbance, the rotor angle strays from nominal into positive changeterritory, reflecting an excess of power generated by SPG 106. When therotor angle increases out of control deadband 204 as shown at 202 b, ESS108 can be controlled to store excess energy from SPG 106, therebyallowing the rotor angle to move back toward nominal. As the frequencyoscillates, the rotor angle may decrease past nominal into negativechange territory, reflecting a lack of power generated by SPG 106. Whenthe rotor angle decreases out of control deadband 204 as shown at 202 c,ESS 108 can be controlled to release stored energy to compensate for thepower deficit of SPG 106, thereby allowing the rotor angle to move backtoward nominal. As long as the rotor angle is within the controldeadband 204, ESS 108 may remain idle without any energy exchange. Insome embodiments, the control deadband 204 may be eliminated or reducedin magnitude, such that any deviation of the rotor angle from nominalresults in energy exchange of ESS 108. Alternatively or additionally,the ESS 108 may provide energy exchange despite the rotor angle beingwithin the control deadband 204, for example, to provide droop controlbased on frequency deviations in electrical network 140 not otherwisereflected in the rotor angle changes (e.g., if SPG 106 is not operatingor operating out of acceptable range).

The hybrid generation system 102 can thus leverage the quick (e.g., lessthan 15 s, and preferably less than is), flexible (e.g., to store orrelease energy), and precise (e.g., to reliably control an amount ofenergy release or stored) energy exchange capabilities of ESS 108 toincrease frequency stability (e.g., as measured by faster recovery ofpower system frequency to nominal following a power system disturbance)by working synergistically with a rotating SPG 106. The combination ofthe ESS 108 with the SPG 106 can extend the effective equal areacriterion for stability limit as compared to the SPG 106 operatingwithout the ESS 108. For example, FIG. 2B is a graph 250 illustratingrotor angle operational limits of an SPG, such as SPG 106. Normally, theSPG has a positive rotor angle limit 252 at +90° and a negative rotorangle limit 258 at −90°. If a power system frequency deviation causedthe rotor angle to exceed either limit 252, 258, protective relays woulddisable the SPG to prevent system damage. However, the dynamic controlof the ESS in response to angle parameters of the SPG increases thedamping effectives of the SPG and extends the equal-area stability limitof the SPG. Thus, the positive rotor angle limit 252 is effectivelyextended to 254 by virtue of charging 256 (e.g., energy storage) of theESS, and the negative rotor angle limit 258 is effectively extended to260 by virtue of discharging 260 (e.g., energy release) of the ESS.

The controlled exchange of energy by ESS 108 can thus amplify thegrid-stabilizing inertial impact of SPG 106 and may be considered to add“synthetic inertia” to the inertia of SPG 106. This synthetic inertia inturn increases the ability of the SPG to mitigate power system frequencydisturbances. Increasing power system stability by adding syntheticinertia from appropriately-controlled ESS 108 can help addressstability-based limitations to adding more renewable resources to powersystems. Electric power system 100 can thus accommodate more V/A powergenerators (e.g., renewables) before hitting stability limits byincreasing the stabilizing impact of a reduced proportion of rotatingSPGs within the system.

As will be appreciated by one of ordinary skill in the art having thebenefit of the present disclosure, the illustration of system 100 inFIG. 1 has been greatly simplified, and practical implementations of thevarious components of system 100 will be more complex. For example,practical implementations of an electrical network 140 could include avariety of transformers, transmission lines, subtransmission lines,substations, and distribution lines. Moreover, although only oneconsumer 150 is illustrated in FIG. 1, the system 100 can have adifferent number of consumers 150 coupled to the network 140 via thesame distribution line 148 or via different distribution lines. Forexample, the network 140 can supply electricity to tens, hundreds,thousands, or millions of consumers 150. Although only five powergeneration systems are illustrated in FIG. 1 (e.g., hybrid system 102,SPGs 142 a, 142 b, and V/A power generators 144 a, 144 b), the system100 can have a different number of power generation systems coupled tothe network 140 via the same transmission lines or via differentdistribution lines. For example, the network 140 can receive electricityfrom tens, hundreds, thousands, or millions of power generation systems.Indeed, as noted above, the provision of hybrid generation systems 102within system 100 can help to stabilize the frequency within electricalnetwork 140, thereby allowing a greater percentage of the powergeneration systems coupled to the electrical network 140 to be V/A powergenerators than would otherwise be possible due to stability limits orregulations.

FIG. 3 shows an exemplary method 300 involving a hybrid generationsystem, for example, hybrid generation system 102. The method 300 canbegin at process block 302, where an ESS is coupled to a rotating SPG.In some embodiments, the SPG is an existing power generator, and the ESSis coupled to the existing SPG to form a hybrid generation system.Alternatively or additionally, the SPG and ESS are constructed orinstalled together as a hybrid generation system. For example, the ESScan be coupled to the SPG in a manner similar to that discussed abovewith respect to ESS 108 and SPG 106 in FIG. 1, e.g., by connecting theSPG 106 and the ESS 108 to a common local bus 126. The ESS and the SPGmay be provided at the same installation or site, so as to be consideredco-located. In some embodiments, process block 302 can also includeconnecting a controller of the ESS to receive signals indicative of atleast on angle parameter (e.g., rotor, torque, and/or power angle) ofthe SPG.

The method 300 can proceed to process block 304, where the ESS isinitialized by storing an initial amount of energy from the SPG and/orfrom the electrical network. For example, a control module of the ESS,such as controller 114, can control the ESS to effect theinitialization. Process block 304 can be such that the amount of energystored in the ESS is less than its maximum capacity, for example, atabout 50% of its capacity. Accordingly, the ESS can be immediatelycapable of either energy storage (e.g., charging) or release (e.g.,discharging). Alternatively, the ESS may be initialized by fullycharging or fully discharging, in which case, the ESS may only becapable of either energy release or energy storage, respectively, at theoutset. Although process block 304 is illustrated as occurring afterprocess block 302, it is also possible for process block 304 to occurbefore process block 302 (e.g., when the ESS is partially or fullycharged prior to coupling to the SPG).

The method 300 can proceed to process block 306, where the SPG isoperated to generate electric power. For example, the rotating SPG canbe at least one of a simple-cycle steam generator, a combustion turbinegenerator, a combined cycle generator, a hydroelectric generator, or adiesel or natural gas engine generator. A rotor within a stator of theSPG can be driven to generate AC electric power, which can then beoutput to a local bus for transmission to the electrical network.

The method 300 can proceed to process block 308, where data indicativeof at least one angle parameter of the SPG during the electric powergeneration is received. For example, the data can be received by acontrol module of the ESS, such as a controller 114. In someembodiments, the data indicative of the at least one angle parameter isreceived via signal(s) directly from one or more sensors that monitor arotor angle of the SPG. Alternatively or additionally, a control moduleof the SPG may monitor rotor angle as part of operation or management ofthe SPG and can send signal(s) to the control module of the ESSproviding the rotor angle data. In some embodiments, the data receivedin process block 308 can instead be data indicative of torque angle orpower angle of the rotating SPG, or can be data of a surrogate variableused to determine or estimate rotor angle, torque angle, or power angle.

The method 300 can proceed to process block 310, where the dataindicative of at least one angle parameter can be compared to a controldeadband to determine if the angle parameter is outside of the controldeadband. The control deadband may provide a buffer around the nominalvalue (e.g., normal operating value) of the angle parameter to avoidunnecessarily responding to normal or expected variations in the angleparameter. For example, the control deadband may be ±1° with respect tothe nominal rotor angle. In other embodiments, the control deadband canbe reduced or eliminated, such that the system can respond to allvariations of the angle parameter. If the angle parameter is determinedto be within the control deadband, the method 300 can proceed to processblock 312, where the ESS is controlled to be idle (e.g., no currentenergy exchange, although it may continue to store previously receivedenergy). The method 300 can thus return to process block 306.

Otherwise, if the at least one angle parameter is determined to beoutside the control deadband at process block 310, the method 300 canproceed to process block 314, where the data indicative of the at leastone angle parameter can be compared to operational limits to determineif the angle parameter is compliant. For example, the operation of theSPG may become unstable when the rotor angle exceeds 90°, and the systemmay consider rotor angles greater than or equal to 90° to benon-compliant. If the angle parameter is determined to be non-compliant,the method 300 can proceed to process block 312, where the ESS is againcontrolled to be idle. The method can then return to process block 306.

Otherwise, if the at least one angle parameter is determined to becompliant at process block 314, the method 300 can proceed to processblock 316, where the ESS is controlled to modulate the power output ofthe hybrid generation system (e.g., the combined outputs of the ESS andthe SPG) based on the data indicative of the least one angle parameter.The power output modulation by the ESS involves release of stored energyto the electrical network or storing energy from the SPG and/or theelectrical network. For example, the energy exchange by the ESS can be afunction of a magnitude of the angle parameter, a difference between themagnitude of the angle parameter and a nominal value for the angleparameter, a rate of change of the angle parameter, or any combinationthereof. In some embodiments, an amount of energy stored in or releasedfrom the ESS is directly proportional to measured rotor angle values orchanges therein. Alternatively or additionally, the control of the ESSenergy exchange can employ PI-based or PID-based control schemes. Othercontrol schemes based on the angle parameter(s) are also possibleaccording to one or more contemplated embodiments.

For example, FIG. 5 shows an exemplary method 500 for control of energyexchange of an ESS based on angle parameter measurements, which method500 can be employed at process block 316 in method 300. The method 500can initiate at process block 502, where it is determined to modulatepower output of the hybrid generation system using the ESS, for example,based on process blocks 310, 314. The method 500 can then proceed toprocess block 504, where it is determined if the at least one angleparameter (e.g., rotor angle) is greater than the control deadband aboutthe nominal angle parameter (e.g., a positive change in rotor angle fromnominal). Alternatively, when there is no control deadband, thedetermination may be with respect to the nominal angle parameter. If theangle parameter is determined to be greater than the control deadband atprocess block 504, the method 500 can proceed to process block 506,where it is determined to store an amount of energy in the ESS.Otherwise, if the angle parameter is less than the control deadband atprocess block 504, the method 500 can proceed to process block 508,where it is determined to release an amount of stored energy from theESS. Note that the option for the angle parameter being within thedeadband is not illustrated in FIG. 5, as the ESS may be controlled tobe at idle (e.g., not modulating the power output) when the angleparameter is within the deadband. As referenced above, the amount ofenergy stored in the ESS or released from the ESS may be a function of(e.g., directly proportional, nonlinear, etc.) of a magnitude of theangle parameter, a magnitude of a change of the angle parameter withrespect to a nominal value, a magnitude of a change of the angleparameter in a given measurement time period, and/or a magnitude of arate of change of the angle parameter. The method 500 can then proceedto process block 510, where the ESS is controlled to store or releasethe determined amounts of energy based on the respective determinationsat either process block 506 or process block 508. In method 500, processblocks 502-510 may be performed by a controller of the ESS, for example,controller 114 in FIG. 1, or by a controller of the hybrid generationsystem shared by the ESS and the SPG.

Returning to FIG. 3, the method 300 can return to process block 306 fromprocess block 316 to repeat process blocks 306-316. Although FIG. 3illustrates a particular order for process blocks 302-316, embodimentsof the disclosed subject matter are not limited thereto. Indeed, incertain embodiments, process blocks may occur in a different order thanillustrated or simultaneously with other process blocks. For example,the generation of power by SPG at process block 306 may be asubstantially continuous process (e.g., interrupted only for periodicmaintenance) and thus can occur at a same time as the receiving data308, determinations 310, 314, and ESS idling 312 or ESS power modulation316.

The control of the ESS to modulate combined power output of the hybridpower generation system based on at least one angle parameter of therotating SPG can provide active damping to stabilize a frequency of thepower produced by the SPG and/or power with an electrical networkconnected to the hybrid power generation system. In some embodiments,the control of the ESS energy exchange based on the angle parameter(s)of the rotating SPG can allow the hybrid generation system to begincompensating for a disturbance before the power system frequencydeviates from nominal or is otherwise detectable. In contrast,conventional systems monitor power frequency (or surrogates forfrequency, such as rotor angle speed) and can only respond once thefrequency deviates from nominal. Thus, the hybrid power generationsystem may be able to respond to respond to disturbances quicker and/orwith fewer oscillations about nominal as compared to conventionalsystems. Moreover, the increased frequency stability offered by thehybrid generation system can allow the electrical network to includemore V/A power generation systems than would otherwise be possible dueto stability limitations or regulations.

In some embodiments, the ESS can also be controlled to store or releaseenergy based on frequency, for example, when the corresponding rotatingSPG is not operating (e.g., offline for maintenance) or when the angleparameter is non-compliant (e.g., greater than or equal to 90°). In suchembodiments, the ESS can be controlled to implement droop control (e.g.,dP/dF, where P is the electrical power and F is frequency). For example,FIG. 4 shows another exemplary method 400 involving a hybrid generationsystem (e.g., system 102) that can provide both angle-parameter-basedcontrol and droop control. The method 400 can begin at process block402, where an ESS is coupled to a rotating SPG to form the hybridgeneration system. The coupling 402 may be in a manner similar to thatdiscussed above for process block 302 in FIG. 3. The method 400 canproceed to process block 404, where the ESS initialized by storing aninitial amount of energy from the SPG and/or from the electricalnetwork. Again, process block 404 may be in a manner similar to thatdiscussed above for process block 304 in FIG. 3, and may occur beforeprocess block 402 (e.g., when the ESS is partially or fully chargedprior to coupling to the SPG). The method 400 can proceed to processblock 406, where the SPG is operated to generate electrical power.Process block 406 may be in a manner similar to that discussed above forprocess block 306 in FIG. 3.

The method 400 can proceed to process block 408, where data indicativeof power system frequency (e.g., frequency of power generated by the SPGand/or power within the electrical network connected to the SPG) isreceived. For example, the power system frequency data can be receivedby a control module of the ESS (such as controller 114), by a controlmodule of the SPG, or by a control module shared by the ESS and SPG. Forexample, the power system frequency data can be based on wide areameasurement systems including synchrophasors. In some embodiments, thepower system frequency data can be generated by one or more sensors thatmonitor power in a component within the electrical network (e.g.,transmission line, subtransmission line, substation, etc.) or atransmission line connecting the SPG to the electrical network. Suchsensors can send a signal to the corresponding control module thatindicates the measured power system frequency. In some embodiments,power system frequency may be independently monitored by a separatesystem, such as a monitor/management system of a power plantinstallation that includes the SPG or a monitor/management system of theelectrical network. The separate system can send a signal to thecorresponding control module that provides an indication of power systemfrequency.

The method 400 can proceed to process block 410, where the dataindicative of power system frequency can be compared to a frequencycontrol deadband to determine if the frequency is outside of thefrequency control deadband. The frequency control deadband may provide abuffer around the nominal value (e.g., normal operating value) of powersystem frequency to avoid unnecessarily responding to normal or expectedvariations in frequency. For example, the frequency control deadband maybe ±0.1 Hz with respect to the nominal frequency (e.g., 60 Hz in NorthAmerica). In other embodiments, the frequency control deadband can bereduced or eliminated, such that the system can respond to allvariations of the power system frequency. If the power system frequencyis determined to be within the frequency control deadband, the method400 can proceed to process block 412, where the ESS is controlled to beidle (e.g., no current energy exchange, although it may continue tostore previously received energy). The method 400 can thus return toprocess block 406.

Otherwise, if the power system frequency is determined to be outside thecontrol deadband at process block 410, the method 400 can proceed toprocess block 414, where data indicative of at least one angle parameterof the SPG during the electric power generation is received. Processblock 414 may be in a manner similar to that discussed above for processblock 308 in FIG. 3. The method 400 can proceed to process block 416,where the angle parameter data can be evaluated to determine if theangle parameter is compliant. For example, the operation of the SPG maybecome unstable when the rotor angle exceeds 90°, and the system mayconsider rotor angles greater than or equal to 90° to be non-compliant.Alternatively or additionally, the data received at process block 414may indicate that the SPG is not operating (e.g., if the SPG isundergoing maintenance). If the angle parameter is determined to becompliant and the SPG is operating, the method can proceed to processblock 418, where the ESS is controlled to operate in a first mode. Inthe first operation mode, the ESS can modulate the power output of thehybrid generation system (e.g., the combined outputs of the ESS and theSPG) based on the data indicative of the at least one angle parameter.Process block 418 may be in a manner similar to that discussed above forprocess block 316 in FIG. 3 and method 500 in FIG. 5.

Otherwise, if the at least one angle parameter is determined to benon-compliant or if the SPG is not operating, the method can proceed toprocess block 420, where the ESS is controlled to operate in a secondmode. In the second operation mode, the ESS can modulate the poweroutput of the hybrid generation system based on the data indicative ofpower system frequency. The power output modulation by the ESS involvesrelease of stored energy to the electrical network or storing energyfrom the SPG and/or the electrical network. The energy exchange by theESS can employ droop control (dP/dF) in a manner similar to conventionalsystems. For example, the ESS can be controlled to release energy to theelectrical network in response to data indicating a decrease infrequency from the nominal value, and the ESS can be controlled to storeenergy from the SPG and/or the electrical network in response to dataindicating an increase in frequency from the nominal value. Othercontrol schemes based on the frequency are also possible according toone or more contemplated embodiments.

In certain embodiments, the method 400 can return from either processblock 418 or process block 420 to process block 406 in order to repeatprocess blocks 406-420. Although FIG. 4 illustrates a particular orderfor process blocks 402-420, embodiments of the disclosed subject matterare not limited thereto. Indeed, in certain embodiments, process stepsmay occur in a different order than illustrated or simultaneously withother process steps. For example, the generation of power by SPG atprocess block 406 may be a substantially continuous process (e.g.,interrupted only for periodic maintenance) and thus can occur at a sametime as the receiving frequency data 408, receiving angle parameter data414, determinations 410, 416, and ESS idling 412 or ESS powermodulations 418, 420.

FIG. 6 shows an exemplary method 600 for control of ESS, for example,control of ESS 108 by controller 114 in FIG. 1. The method 600 includesan act 602 of controlling storage of energy in or release of storedenergy from the ESS based on a measurement of at least one angleparameter (e.g., rotor angle, torque angle, and/or power angle) of anelectrical power generator, for example, rotating SPG 106 in FIG. 1. Forexample, ESS control 602 can be in a manner similar to that describedabove with respect to any of FIGS. 3-5.

In the above discussed embodiments and examples, the controlled exchangeof energy by the ESS with the electrical network can amplify thegrid-stabilizing inertial impact of the associated SPG, therebyimproving recovery of the power system frequency following adisturbance. The ESS control scheme based on angle parameter(s) of theSPG can, in theory, provide as much as a 26-fold increase in systemfrequency damping (e.g., reduction in unwanted oscillations aroundnominal frequency) as compared to the SPG alone. The controlled powerexchange by the ESS can extend the stability range of the rotating SPG,based on the equal area curve criteria. If the equal area curve isexceeded, the power angle of the SPG will exceed 90°. Typically,out-of-step protective relays will trip (e.g., disable) the SPG beforethis limit is reached. However, by extending the equal-area region ofthe SPG and accordingly its stable operating regime, grid-connected SPGswith their stabilizing inertia can extend fault ride through (FRT)operations when needed most for system frequency deviation recovery,e.g., when they would otherwise trip before losing synchronism with theperturbed and oscillating power system.

FIGS. 7A-7B show results of a simulation of post-fault behavior of ahybrid generation system according to an embodiment of the disclosedsubject matter. The simulation employed a single SPG coupled to a singleESS via a common local bus, which was connected to an infinite bussystem via three transmission lines. Parameters for the simulationincluded 0.24 pu for the SPG transient reactance, 0.1 pu for thetransformer reactance, 0.4 pu for the reactance per transmission line,0.1 pu for the power system reactance, and 4 for the SPG inertiaconstant. The simulation included the following sequence of events: (1)in a pre-fault condition, the system operates at nominal frequency; (2)the system experiences a fault on one of the three transmission lines;and (3) after the fault clears at 0.15 s, two of the transmission linesand the resources (ESS, SPG, infinite bus) remain in service.

Without ESS damping control, the energy in the rotor of the SPGoscillates and the overall response is underdamped. With ESS dampingcontrol enabled, the overall system is positively damped. Theoscillatory behavior of the modeled SPG is shown in the rotor angleprofile 700 shown in FIG. 7A. The ESS is controlled to transfer thegenerator's energy from the positive angle change regions to thenegative angle change regions so as to dampen the post-faultoscillations. To obtain such a response, the energy storage in (e.g.,charging) and energy transfer from (e.g., discharging) the ESS iscontrolled in proportion to the change in SPG rotor angle. The resultingmanaged power output profile 750 is illustrated in FIG. 7B. The overallresult from dynamic control of the ESS energy exchange is an improvementin recovery of the modeled power system's frequency following adisturbance.

In some embodiments, the ESS energy exchange to stabilize frequencyoscillations in response to a disturbance can also act to compensatelocal voltage variations (e.g., in the local bus of the hybridgeneration system or a small area of the electrical network coupled tothe local bus). For example, if a variation in the power systemfrequency is due to a disturbance where transient load is greater thanpower generation, the energy exchange by the ESS to stabilize frequencycan offset or otherwise mitigate a local voltage dip resulting from thatdisturbance. Other system benefits may also result from the power systemfrequency stabilization offered by the ESS energy exchange.

Although much of the discussion above has focused on rotor angle of therotating SPG as the angle parameter for control of the ESS, torque angleor power angle of the rotating SPG can instead be used for control ofthe ESS. Accordingly, in the instances above describing control of theESS based on rotor angles, such description also includes control basedon torque angle and/or power angle, even if not specifically recited.

In some embodiments, the ESS can be used to add power-system stabilizingattributes to a variable or asynchronous (V/A) power generator, such asa photovoltaic or wind-turbine systems. In such embodiments, the V/Apower generator would lack a rotor angle that can serve as a basis forcontrol of the ESS energy exchange. However, the ESS can be controlledto provide energy exchange with the V/A power generator based on powersystem frequency (e.g., employing droop control) or based on a powerangle (e.g., the angle between the voltage and current). Frequencyresponsive output from the V/A power generator alone may requiresub-optimal operation of the underlying renewable resource for provisionof power, thus wasting clean energy. Hybridizing the V/A power generatorwith an ESS, which is controlled for frequency response with activedamping, can allow the wind or solar resource to deliver more netenergy, while delivering grid-supportive inertia-equivalent (syntheticinertia) capability.

FIG. 8 depicts a generalized example of a suitable computing environment800 in which the described innovations may be implemented. The computingenvironment 800 is not intended to suggest any limitation as to scope ofuse or functionality, as the innovations may be implemented in diversegeneral-purpose or special-purpose computing systems. For example, thecomputing environment 800 is any of a variety of computing devices(e.g., desktop computer, laptop computer, server computer, tabletcomputer, etc.).

The computing environment 800 includes one or more processing units 810,815 and memory 820, 825. In FIG. 8, this basic configuration 830 isincluded within a dashed line. The processing units 810, 815 executecomputer-executable instructions. Each processing unit can be ageneral-purpose central processing unit (CPU), processor in anapplication-specific integrated circuit (ASIC) or any other type ofprocessor. In a multi-processing system, multiple processing unitsexecute computer-executable instructions to increase processing power.For example, FIG. 8 shows a central processing unit 810 as well as agraphics processing unit or co-processing unit 815. The tangible memory820, 825 may be volatile memory (e.g., registers, cache, RAM),non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or somecombination of the two, accessible by the processing unit(s). The memory820, 825 stores software 880 implementing one or more innovationsdescribed herein, in the form of computer-executable instructionssuitable for execution by the processing unit(s).

A computing system may have additional features. For example, thecomputing environment 800 includes storage 840, one or more inputdevices 850, one or more output devices 860, and one or morecommunication connections 870. An interconnection mechanism (not shown)such as a bus, controller, or network interconnects the components ofthe computing environment 800. Typically, operating system software (notshown) provides an operating environment for other software executing inthe computing environment 800, and coordinates activities of thecomponents of the computing environment 800.

The tangible storage 840 may be removable or non-removable, and includesmagnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any othermedium which can be used to store information in a non-transitory way,and which can be accessed within the computing environment 800. Thestorage 840 stores instructions for the software 880 implementing one ormore innovations described herein.

The input device(s) 850 may be a touch input device such as a keyboard,mouse, pen, or trackball, a voice input device, a scanning device, oranother device that provides input to the computing environment 800. Theoutput device(s) 860 may be a display, printer, speaker, CD-writer, oranother device that provides output from computing environment 800.

The communication connection(s) 870 enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,audio or video input or output, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia can use an electrical, optical, RF, or other carrier.

Some embodiments of the disclosed methods can be performed usingcomputer-executable instructions implementing all or a portion of thedisclosed technology in a computing cloud 890. For example, thedisclosed methods can be executed on processing units 810, 815 locatedin the computing environment 830 and/or on servers located in thecomputing cloud 890.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed methods can be used in conjunction with other methods.

Any of the disclosed methods can be implemented as computer-executableinstructions stored on one or more computer-readable storage media(e.g., one or more optical media discs, volatile memory components (suchas DRAM or SRAM), or non-volatile memory components (such as flashmemory or hard drives)) and executed on a computer (e.g., anycommercially available computer, including smart phones or other mobiledevices that include computing hardware). As used herein, the termcomputer-readable storage media does not include communicationconnections, such as signals, carrier waves, or other transitorysignals. Any of the computer-executable instructions for implementingthe disclosed techniques as well as any data created and used duringimplementation of the disclosed embodiments can be stored on one or morecomputer-readable storage media. The computer-executable instructionscan be part of, for example, a dedicated software application or asoftware application that is accessed or downloaded via a web browser orother software application (such as a remote computing application).Such software can be executed, for example, on a single local computer(e.g., any suitable commercially available computer) or in a networkenvironment (e.g., via the Internet, a wide-area network, a local-areanetwork, a client-server network (such as a cloud computing network), orother such network) using one or more network computers.

For clarity, only certain selected aspects of the software-basedimplementations are described. Other details that are well known in theart are omitted. For example, it should be understood that the disclosedtechnology is not limited to any specific computer language or program.For instance, aspects of the disclosed technology can be implemented bysoftware written in C++, Java, Perl, any other suitable programminglanguage. Likewise, the disclosed technology is not limited to anyparticular computer or type of hardware. Certain details of suitablecomputers and hardware are well known and need not be set forth indetail in this disclosure.

It should also be well understood that any functionality describedherein can be performed, at least in part, by one or more hardware logiccomponents, instead of software. For example, and without limitation,illustrative types of hardware logic components that can be used includeField-programmable Gate Arrays (FPGAs), Program-specific IntegratedCircuits (ASIC s), Program-specific Standard Products (ASSPs),System-on-a-chip systems (SOCs), Complex Programmable Logic Devices(CPLDs), etc.

Furthermore, any of the software-based embodiments (comprising, forexample, computer-executable instructions for causing a computer toperform any of the disclosed methods) can be uploaded, downloaded, orremotely accessed through a suitable communication means. Such suitablecommunication means include, for example, the Internet, the World WideWeb, an intranet, software applications, cable (including fiber opticcable), magnetic communications, electromagnetic communications(including RF, microwave, and infrared communications), electroniccommunications, or other such communication means.

As used in this application the singular forms “a,” “an,” and “the”include the plural forms unless the context clearly dictates otherwise.Additionally, the term “includes” means “comprises.” Further, the term“coupled” encompasses mechanical, electrical, magnetic, optical, as wellas other practical ways of coupling or linking items together, and doesnot exclude the presence of intermediate elements between the coupleditems. Furthermore, as used herein, the term “and/or” means any one itemor combination of items in the phrase.

The disclosed systems, methods, and apparatus are not limited to anyspecific aspect or feature or combinations thereof, nor do the disclosedthings and methods require that any one or more specific advantages bepresent or problems be solved. Additionally, the description sometimesuses terms like “produce,” “provide,” “control,” “receive,” “evaluate,”and “determine” to describe the disclosed methods. These terms arehigh-level descriptions of the actual operations that are performed. Theactual operations that correspond to these terms will vary depending onthe particular implementation and are readily discernible by one ofordinary skill in the art having the benefit of the present disclosure.

Theories of operation, scientific principles, or other theoreticaldescriptions presented herein in reference to the apparatus or methodsof this disclosure have been provided for the purposes of betterunderstanding and are not intended to be limiting in scope. Theapparatus and methods in the appended claims are not limited to thoseapparatus and methods that function in the manner described by suchtheories of operation.

In view of the many possible embodiments to which the principles of thedisclosed technology may be applied, it should be recognized that theillustrated embodiments are only examples and should not be taken aslimiting the scope of the claimed subject matter. We therefore claim asour invention all that comes within the scope of these claims.

1. An apparatus comprising: an energy storage system coupled to anoutput of an electrical power generator; and a control system having asignal output coupled to the energy storage system that causes theenergy storage system to store or release energy based on at least oneangle parameter, each angle parameter being selected from rotor angle,torque angle, and power angle of the electrical power generator.
 2. Theapparatus of claim 1, wherein the control system comprises: one or moreprocessors; an input/output interface situated to control the energystorage system; and computer-readable storage storingcomputer-executable instructions that, when executed by the one or moreprocessors, cause the one or more processors to collectively control theenergy storage system to store or release energy based on the at leastone angle parameter.
 3. The apparatus of claim 1, wherein the controlsystem is configured to control energy storage or release by the energystorage system to provide active frequency damping of electrical poweroutput.
 4. The apparatus of claim 1, wherein the control system isconfigured to control energy storage or release by the energy storagesystem so as to stabilize a frequency of electrical power output at anominal value or within a frequency range around the nominal value. 5.The apparatus of claim 1, wherein: the energy storage system comprisesan inverter or flexible alternating current transmission system (FACTS)and at least one of a battery, a capacitor, a flywheel, a stationarypower source, a pumped fluid storage, or a thermal energy storage; andthe electrical power generator comprises at least one of a steamgenerator, a combustion turbine generator, a combined cycle generator, ahydroelectric generator, or a diesel or natural gas engine generator. 6.The apparatus of claim 1, wherein the control system has a signal inputcoupled to the electrical power generator to receive sensor signalsindicative of the at least one angle parameter.
 7. The apparatus ofclaim 1, wherein the control system is configured to: control the energystorage system to store energy in response to a positive change overtime of the at least one angle parameter; and control the energy storagesystem to release stored energy in response to a negative change overtime of the at least one angle parameter.
 8. The apparatus of claim 1,wherein the control system is configured to control the energy storagesystem such that an amount of energy stored or released is a function ofat least one of: a magnitude of the at least one angle parameter; adifference between the magnitude of the at least one angle parameter anda nominal value; or a rate of change of the at least one angleparameter.
 9. The apparatus of claim 1, wherein the energy storagesystem is coupled to the output of the electrical power generator via alocal bus.
 10. A method for operating an energy storage system coupledto an electrical power generator, the method comprising: in a firstoperation mode, controlling storage of energy in or release of storedenergy from the energy storage system based on a measurement of at leastone angle parameter, the angle parameter being selected from rotorangle, torque angle, and power angle of the electrical power generator.11. The method of claim 10, wherein the controlling provides activefrequency damping of electrical power from the electrical powergenerator.
 12. The method of claim 10, wherein the controlling is suchthat a frequency of electrical power output from a combination of theenergy storage system and the electrical power generator is stabilizedat a nominal value or within a frequency range around the nominal value.13. The method of claim 10, wherein the energy storage system comprises:an inverter or flexible alternating current transmission system (FACTS);and at least one of a battery, a capacitor, a flywheel, a stationarypower source, a pumped fluid storage, or a thermal energy storage. 14.The method of claim 10, further comprising: receiving at least onesensor signal from the electrical power generator or a power managementunit thereof, wherein the at least one sensor signal is indicative ofthe at least one angle parameter, or is indicative of changes of the atleast one angle parameter over time, wherein the controlling isresponsive to the at least one sensor signal.
 15. The method of claim10, wherein the controlling comprises: controlling the energy storagesystem to store energy in response to a positive change of the at leastone angle parameter during a measurement time interval; and controllingthe energy storage system to release stored energy in response to anegative change of the at least one angle parameter during themeasurement time interval.
 16. The method of claim 10, wherein thecontrolling is such that an amount of energy stored in or released fromthe energy storage system is a function of at least one of: a magnitudeof the at least one angle parameter; a difference between the magnitudeof the at least one angle parameter and a nominal value; or a rate ofchange of the at least one angle parameter.
 17. The method of claim 10,further comprising: in a second operation mode, controlling the storageof energy in or release of stored energy from the energy storage systembased on power frequency variations in an electrical network coupled tothe electrical power generator, wherein the energy storage system isoperated in the second operation mode in response to an absence of themeasurement of the at least one angle parameter, or to the measurementof the at least one angle parameter being outside a predetermined range.18. The method of claim 17, wherein, in the second operation mode, thecontrolling is such that an amount of energy stored in or released fromthe energy storage system is proportional to a frequency of the power inthe electrical network or a difference between the frequency of thepower and a nominal value.
 19. The method of claim 10, furthercomprising: prior to initiation of the first operation mode, determiningthat a frequency of power in an electrical network coupled to theelectrical power generator is outside of predetermined range.
 20. Themethod of claim 10, wherein the electrical power generator is asynchronous power generator comprising at least one of a simple cyclesteam generator, a combustion turbine generator, a combined cyclegenerator, a hydroelectric generator, or a diesel or natural gas enginegenerator.
 21. A control system comprising: one or more processors; andcomputer-readable storage media storing computer-readable instructionsthat, when executed by the one or more processors, cause the one or moreprocessors to perform the method of claim
 10. 22. The control system ofclaim 21, wherein the instructions stored by the computer-readablestorage media comprise: instructions that cause the one or moreprocessors to receive one or more sensor signals indicative of the atleast one angle parameter, or of changes in the at least one angleparameter over a time period; instructions that cause the one or moreprocessors to generate one or more first control signals for the energystorage system that cause the energy storage system to store energy inresponse to a positive change in the at least one angle parameter duringthe time period; and instructions that cause the one or more processorsto generate one or more second control signals for the energy storagesystem that cause the energy storage system to release stored energy inresponse to a negative change in the at least one angle parameter duringthe time period.
 23. A hybrid power generation system comprising: anelectrical power generator configured to produce alternating current(AC) electrical power for an electrical network; and a frequency dampingunit coupled to the electrical power generator, the frequency dampingunit comprising an energy storage system and a controller, wherein thecontroller is configured to control the energy storage system tomodulate a combined power output of the electrical power generator andthe energy storage system based on at least one angle parameter, eachangle parameter being selected from rotor angle, torque angle, and powerangle of the electrical power generator.
 24. The hybrid power generationsystem of claim 23, wherein: the electrical power generator has aphysical inertia; and the frequency damping unit is configured to addsynthetic inertia to the physical inertia of the electrical powergenerator.
 25. The hybrid power generation system of claim 24, whereinthe synthetic inertia added by the frequency damping unit acts tostabilize a frequency of the combined power output at a nominal value ofthe electrical network or within a frequency range around the nominalvalue.
 26. The hybrid power generation system of claim 23, wherein aneffective equal area criterion for stability limit of the hybrid powergeneration system is greater than an effective equal area criterion forstability limit of the electrical power generator alone.
 27. The hybridpower generation system of claim 23, further comprising: one or moresensors configured to measure the at least one angle parameter and togenerate one or more sensor signals in response to the at least onemeasured angle parameter, wherein the controller is configured tocontrol the energy storage system based on the one or more sensorsignals.
 28. The hybrid power generation system of claim 23, wherein theelectrical power generator is a synchronous power generator comprisingat least one of a simple cycle steam generator, a combustion turbinegenerator, a combined cycle generator, a hydroelectric generator, adiesel engine generator, or a natural gas engine generator.
 29. Thehybrid power generation system of claim 23, wherein the energy storagesystem comprises: an inverter or flexible alternating currenttransmission system (FACTS); and at least one of a battery, a capacitor,a flywheel, a stationary power source, a pumped fluid storage, or athermal energy storage.
 30. The hybrid power generation system of claim23, wherein the controller is configured to: control the energy storagesystem to store energy in response to a positive change of the at leastone angle parameter during a time period; and control the energy storagesystem to release stored energy in response to a negative change of theat least one angle parameter during the time period.
 31. The hybridpower generation system of claim 23, wherein the controller isconfigured to control the energy storage system such that an amount ofenergy stored in or released from the energy storage system is afunction of at least one of: a magnitude of the at least one angleparameter of the electrical power generator; a difference between themagnitude of the at least one angle parameter and a predetermined value;or a rate of change of the at least one angle parameter.
 32. The hybridpower generation system of claim 23, wherein: the frequency damping unitis coupled to the electrical power generator by one or more local buses;and the electrical power generator is coupled to the electrical networkby one or more power transmission lines.
 33. An electrical power systemcomprising: one or more of the hybrid power generation systems of claim23; one or more variable or asynchronous (V/A) power generation systems;and an electrical network coupled to the hybrid and V/A power generationsystems and configured to transmit power from the power generationsystems to one or more end users.
 34. The electrical power system ofclaim 33, wherein: in each hybrid power generation system, the frequencydamping unit is coupled to the corresponding electrical power generatorby one or more local buses; and each of the hybrid and V/A powergeneration systems is coupled to the electrical network by one or morerespective power transmission lines.
 35. The electrical power system ofclaim 33, wherein the one or more V/A power generation systems comprisesat least one of a wind turbine or a photovoltaic device.